Optical fiber in its simplest form, is composed of two concentric regions of glass. The inner region is known as the core and the outer region is known as the cladding. A plastic coating known as the buffer is typically applied to the fiber and surrounding the cladding for protection of the fiber.
The index of refraction of the core is greater than the index of refraction of the cladding, thus, light introduced to the fiber via a free end is guided by total internal reflection. The propagating mode is contained in the core and non-propagating evanescent fields, which decay rapidly toward zero density radially from the core-cladding boundary, reside in the cladding. Furthermore the cladding, due to its thickness, for all intents and purposes, totally contains the evanescent fields.
The propagating mode in the core, can be affected by perturbing the evanescent field in the cladding. By doing so, useful devices can be fabricated out of fiber. One particularly useful device for fiber optic gyro (FOG) applications is known as an evanescent coupler. The evanescent coupler relies on the evanescent fields in one fiber to excite a propagating mode in another fiber, thereby coupling energy from one fiber to another.
Because the evanescent fields decay rapidly from the core-cladding interface, it is necessary to remove most of the cladding in a small area from both fibers, so that the interaction may take place. Removing the cladding can be accomplished by a glass polishing procedure The first step in the polishing procedure would be to strip some buffer material in the middle of a length of fiber. Stripping can be accomplished with a commercially available, heated filament, wire stripper The fiber would then be mounted on a curved surface The curved surface or substrate could be machined ceramic block for example, configured with one face to which the optical fiber can be attached. Attaching the fiber to the substrate can be accomplished with an adhesive, such as an epoxy. The fiber-substrate assembly is mounted to the oscillating arm of a polishing machine. The machine polishing wheel would typically have a fine diamond abrasive pad affixed to it and the polishing slurry would drip onto the pad and be spread around by the fiber as it is polished.
At the completion of the polishing operation, the fiber will have an optical flat surface polished on to it. A coupler is then made by joining two such structures at the flat region. To achieve a given coupling performance, the polishing must stop at a specified distance from the core. In typical production fiber, the core tends to wander slightly within the cladding. Therefore, the distance from the core to the cladding at the end of a fixed time polish operation, would vary from one fiber length to another fiber length. Also the distance of the flat from the core at the end of the polishing process needs to be something on the order of a few microns. Therefore, not only is there no way to make a physical thickness measurement during the above described process to determine distance from the core, but there would be no way to develop an empirical scheme based on grind time to within the accuracy required.
Currently, the proximity to the core from the optical flat is determined by periodically removing the fiber and substrate from the polishing machine, launching light into the fiber, looking at the output on a power meter, and performing an oil drop test. The oil used is an index of refraction formulated optical oil chosen to have a greater index of refraction than the core of the fiber. As more cladding is removed, subsequently higher evanescent field density is exposed. If the cladding is polished down to the proximity of the core and the oil is applied to the polished region, a power drop will be noted on the power meter since a portion of the light will couple out of the fiber due to the interaction between the evanescent field and the oil. The closer the core is to the polished region, the greater the power drop will be. An empirical polishing procedure can then be formulated based on the results of the oil drop test. This current methodology is time consuming and can not be automated
Another method is described in U.S. Pat. No. 4,630,884 which teaches a method and apparatus for monitoring and polishing an optical fiber where the polishing is periodically or occasionally stopped to take readings of coherent light then communicated into the fluid waveguide between the polishing surface and the fiber and coupled from the fluid into the fiber. A light intensity monitor coupled to one end of the fiber measures light coupled into the fiber from the light source through the fluid waveguide. Also taught is the use of a long chain polymer such as polypropylene to improve laminar flow of the transparent fluid constituting the fluid waveguide. Thus, the method of this patent requires interruption of the lapping or polishing process to obtain readings of the extent the polishing has worked to produce fiber cladding removal. This interruption prevents the process from being efficiently automated to produce the faster, reliable and repeatable results of the subject invention. Also, the lubricating fluid, with or without the addition of any additive to improve laminar flow is temperature sensitive. Thus, light coupling through the fluid waveguide into the optical fiber will vary from procedure to procedure and during each procedure. Thus, relying on the fluid waveguide to communicate the light coupled into or out of the core and used as the basis for measuring the progress of the lapping or polishing process can introduce uncertainty and unreliability.